Calculation of soft decision values using reliability information of the amplitude

ABSTRACT

Method of, and arrangement and device for, decoding a communications signal in a digital communications system, where the communications signal is modulated according to a modulation scheme including amplitude information; generating a likelihood value for a received communications signal, decoding the communications signal based on at least the generated likelihood value, providing a reliability indication of the amplitude information conveyed by the received communications signal wherein the step of generating the likelihood value further comprises generating the likelihood value on the basis of the provided reliability indication of the amplitude information.

The patent application claims the benefit of priority from U.S.Provisional Patent Application Ser. No. 60/429,579 filed on Nov. 26,2002. This application incorporates by reference the entire disclosureof U.S. Provisional Patent Application Ser. No. 60/429,579.

FIELD OF THE INVENTION

This invention relates to digital communications systems and, moreparticularly, the generation of soft reliability values for multilevelsignals.

BACKGROUND

Within the field of digital communications, multilevel modulation isused to map a number of bit sequences to a signal alphabet comprising anumber of signal symbols, i.e. a number of points in signal space. Forexample, a bit sequence may be mapped onto a point in a complex signalspace. A signal alphabet of size M allows log₂(M) bits to be mapped toeach symbol. However, when symbols are received at a receiver, they maybe affected by noise, thereby affecting the decoding of the signal whenretrieving the transmitted bit sequence. If multilevel modulation isused in conjunction with channel coding, many channel decoders, such asiterative decoders based on the BCJR algorithm, require likelihoodvalues for the received bits, so-called soft values, as an input. A softvalue corresponds to a likelihood value of a single bit being 0 or 1.

Examples of multilevel modulation include multi-amplitude levelmodulation in Pulse Amplitude Modulation (PAM), multi signal pointmodulation in Quadrature Amplitude Modulation (QAM), or the like.

For example, an emerging technology for wideband digital radiocommunications of Internet, multimedia, video and othercapacity-demanding applications in connection with the third generationof mobile telephone systems is the evolving Wideband Code DivisionMultiple Access (WCDMA) specified as part of the 3GPP standardisationorganisation. Within this technology, High Speed Downlink Packet Access(HSDPA) is provided including a high speed downlink shared channel(HS-DSCH) which uses 16-QAM. In 16-QAM, M=16, i.e. each symbol in thesignal alphabet represents 4 bits. Future releases may comprise evenlarger constellation sizes such as 64-QAM. Unlike QPSK, 16-QAM alsoincludes amplitude information into the modulation.

However, in modulation schemes such as 16-QAM that include amplitudeinformation in the modulation, it is a problem that the quality of thesignal decoding is sensitive to amplitude distortions.

SUMMARY

The above and other problems are solved when a method of decoding acommunications signal in a digital communications system, where thecommunications signal is modulated according to a modulation schemeincluding amplitude information; the method comprising

-   -   generating a likelihood value for a received communications        signal;    -   decoding the communications signal based on at least the        generated likelihood value;        is characterised in    -   that the method further comprises providing a reliability        indication of the amplitude information conveyed by the received        communications signal ; and    -   that the step of generating the likelihood value further        comprises generating the likelihood value on the basis of the        provided reliability indication of the amplitude information.

Consequently, by providing reliability information about the amplitudeinformation, and including this reliability information in thecalculation of the likelihood values, reliable amplitude information maybe distinguished from unreliable amplitude information, therebyincreasing the accuracy of the calculated likelihood values and, thus,improving the quality of the signal decoding considerably.

It is a further advantage of the invention that the calculatedlikelihood values are less sensitive to amplitude distortions.

In one embodiment, the likelihood value is indicative of a likelihoodthat the received communications signal represents a bit sequencecomprising a predetermined bit value at a predetermined position. Hence,preferably, a likelihood value is calculated for each bit of thetransmitted bit sequence. In one embodiment, the likelihood values aresoft values for use by a decoder based on soft values, e.g. a Viterbidecoder, turbo decoder, BCJR decoder, or the like.

According to a preferred embodiment of the invention, the method furthercomprises receiving the communications signal by a receiver module, andthat the reliability indication is provided by the receiver module.

It has been realised by the inventors that reliability information aboutthe amplitude information is often available at the receiver and, thus,may be utilised in the subsequent baseband processing. For instance, again change in the receiver may be induced by the terminal, e.g. inorder to adjust for the signal strength. Such a gain change may not, orat least only partially, be correctable by the receiver due tocalibration and synchronisation issues.

Hence, according to a further preferred embodiment of the invention, thestep of receiving the communications signal further comprises scalingthe communications signal by an amplifier and the step of providing thereliability indication comprises generating the reliability indicationon the basis of a gain setting of said amplifier. Hence, by forwardinginformation about the occurred gain change from the receiver to thebaseband processing, the baseband system may incorporate thisinformation in the calculation of soft values, thereby providing anefficient handling of receiver introduced amplitude distortions, sincereliable and unreliable values are distinguished.

It is a further advantage of the invention that it provides a simple andcost-effective method of reducing the effects of amplitude distortions,in particular receiver introduced amplitude distortions.

Preferably, the step of generating the likelihood value on the basis ofthe provided reliability indication comprises determining whether anamplitude change by a predetermined magnitude has occurred within apredetermined time period. Hence, a simple mechanism is provided fordetermining whether or not the amplitude information is regarded asreliable. This is particularly advantageous since in many situations acomplete model of the amplitude distortion is not available.Furthermore, this algorithm allows for a simple and cost-effectiveimplementation which does not require many computational resources.Here, the term amplitude change by a predetermined magnitude comprisesan amplitude change that is larger than a predetermined threshold.

The adjustment of the receiver's gain due to a varying signal level maybe larger than a predetermined threshold. For example, in a receiverwhere the amplifier gain may be changed between a number of discretelevels, an amplitude change by a predetermined magnitude may correspondto any change of amplifier gain settings or to a gain change which islarger than a predetermined threshold. Similarly, if the gain may bechanged continuously or quasi-continuously, a change in amplitude by apredetermined magnitude may be detected as a gain change larger than apredetermined threshold.

Preferably, the predetermined time period corresponds to a time slot forcommunicating a bit sequence. Hence, it is detected whether a gainchange has occurred during reception of a signal symbol representing abit sequence. As such a gain change may cause amplitude distortionsduring reception of a signal symbol, the decoding of the correspondingbits of this symbol are less reliable.

In a further preferred embodiment, the step of generating the likelihoodvalue on the basis of the provided reliability indication furthercomprises adjusting the likelihood value to a value corresponding tohigher uncertainty of a predetermined bit value if an amplitude changehas occurred and if said bit value is encoded using amplitudeinformation. In many modulation schemes, e.g. in the 16-QAM schemeproposed for the above-mentioned HSDPA, not all bits of a signal symboldepend on amplitude information. Hence, by limiting the adjustment oflikelihood values on the basis of amplitude information to those bitswhich actually depend on amplitude information, the efficiency of thesystem is increased without reducing the quality of decoding.

In one embodiment where a likelihood value of zero corresponds tomaximum uncertainty, a likelihood value that is known to be affected byamplitude distortions may be adjusted such that its absolute value isreduced, e.g. the likelihood value may be set to zero.

Hence, in a yet further preferred embodiment, the modulation scheme is16-QAM wherein each signal symbol comprises four bits and where twopredetermined bits of said four bits depend on amplitude information;and the method comprises adjusting the likelihood values of said twopredetermined bits to a value corresponding to higher uncertainty, if achange in amplitude is detected during the time slot for communicatingsaid four bit sequence.

In many receivers a number of bit sequences, corresponding to a numberof consecutive time slots, are decoded together.

Hence, in a preferred embodiment of the invention, the method furthercomprises rejecting all received signal symbols received within a timeinterval comprising a predetermined number of consecutive slots, if achange in amplitude by a predetermined magnitude is detected in morethan a predetermined fraction of slots of said number of consecutiveslots. Consequently, a number of signal symbols may be rejected asunreliable before the actual decoding stage, thereby speeding up thedecoding process and, thus, increasing the throughput of the receiver.

In one embodiment of the invention, the method further comprises thestep of providing the reliability value as an input to a decoder, e.g.an iterative decoder using the BCJR algorithm or any other decoder usingsoft values as an input. It is an advantage of the invention that itprovides an accurate and resource-efficient soft-value approximationwhich results in soft values that are less sensitive to amplitudedistortions.

The signal space may comprise one or more dimensions. For example, inQAM modulation two amplitude-modulated signals are transmitted on asingle carrier, but shifted in phase by 90 degrees. Hence, the resultingsignal points may be represented in the complex plane representing theso-called in-phase (I) and quadrature (Q) components of the QAM signal.

The present invention can be implemented in different ways including themethod described above and in the following, an arrangement, and furtherproduct means, each yielding one or more of the benefits and advantagesdescribed in connection with the first-mentioned method, and each havingone or more preferred embodiments corresponding to the preferredembodiments described in connection with the first-mentioned method anddisclosed in the dependant claims.

It is noted that the features of the method described above and in thefollowing may be implemented in software and carried out in a dataprocessing system or other processing means caused by the execution ofcomputer-executable instructions. The instructions may be program codemeans loaded in a memory, such as a RAM, from a storage medium or fromanother computer via a computer network. Alternatively, the describedfeatures may be implemented by hardwired circuitry instead of softwareor in combination with software.

The invention further relates to an arrangement for decoding acommunications signal in a digital communications system, where thecommunications signal is modulated according to a modulation schemeincluding amplitude information; the arrangement comprising

-   -   processing means adapted to generate a likelihood value for a        received communications signal;    -   a decoder for decoding the communications signal based on at        least the generated likelihood value;    -   means for providing a reliability indication of the amplitude        information conveyed by the received communications signal; and        the processing means is further adapted to generate the        likelihood value on the basis of the provided reliability        indication of the amplitude information.

The term processing means comprises any suitable general- orspecial-purpose programmable microprocessors, Digital Signal Processors(DSP), Application Specific Integrated Circuits (ASIC), ProgrammableLogic Arrays (PLA), Field Programmable Gate Arrays (FPGA), specialpurpose electronic circuits, etc., or a combination thereof.

The means for providing a reliability indication of the amplitudeinformation conveyed by the received communications signal may compriseany suitable circuitry, processor, or the like, adapted to determine asuitable measure of reliability of the amplitude information and togenerate a corresponding reliability indication, e.g. a reliabilitysignal, or the like.

In a preferred embodiment of the invention, the arrangement comprises

-   -   a receiver for receiving a communication signal;    -   an amplifier for scaling the received communications signal        according to a predetermined amplifier gain;    -   a gain control module for controlling the amplifier gain        according to a received signal strength, the gain control module        being adapted to feed a gain control signal to the amplifier;        and    -   a control unit for generating amplitude information, the control        unit being adapted to receive the gain control signal from the        gain control unit and to generate an amplitude information        signal; and the processing means is adapted to receive the        amplitude information signal from the control unit.

The receiver, e.g. a radio receiver or the like, may comprise anysuitable circuitry or device for receiving the communications signal.

In other embodiments the reliability information may be provided byother means, such as a detector for detecting amplitude variations inthe transmission channel, or the like.

Further preferred embodiments of the arrangement according to theinvention correspond to the preferred embodiments described inconnection with the first-mentioned method and disclosed in thedependant claims.

The invention further relates to a device for receiving a communicationssignal comprising an arrangement as described above and in thefollowing.

The device may be any electronic equipment or part of such electronicequipment, where the term electronic equipment includes computers, suchas stationary and portable PCs, stationary and portable radiocommunications equipment. The term portable radio communicationsequipment includes mobile radio terminals such as mobile telephones,pagers, communicators, e.g. electronic organisers, smart phones, PDAs,or the like.

For example, in a cellular communications system an arrangementaccording to the invention may be included in a mobile terminal and/or abase station of the cellular communications system.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be explained more fully below in connection withpreferred embodiments and with reference to the drawings, in which:

FIG. 1 schematic illustrates a general model of a communications system;

FIG. 2 shows an example of a signal constellation with 16 signalsymbols;

FIG. 3 shows a block diagram of a receiver according to an embodiment ofthe invention;

FIG. 4 shows a more detailed block diagram of a receiver according to afirst embodiment of the invention;

FIG. 5 shows a more detailed block diagram of a receiver according to asecond embodiment of the invention;

FIG. 6 shows a flow diagram of a method according to an embodiment ofthe invention;

FIG. 7 shows a flow diagram of a method according to another embodimentof the invention; and

FIG. 8 illustrates the method of FIG. 7 with reference to an example ofa received communications signal.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a general model of a communicationssystem. The communications system comprises a transmitter 101 and areceiver 102 communicating via a communications channel 103. Forexample, in an actual implementation the transmitter may be a mobileterminal and the receiver a base station of a cellular radio frequency(RF) communications system or vice versa. The mobile terminal and thebase station communicate with each other via communications signalstransmitted over an air interface. For the purpose of the followingdescription, the transmitter 101 is considered to comprise a modulator105 which applies the necessary modulation to the signal so that it canbe transmitted over the communications channel. The receiver comprises ademodulator 106 implementing a demodulation process corresponding to themodulation process implemented by the modulator 105, thereby allowing torecover the originally transmitted information from the received signal.In many modulation schemes, e.g. quadrature amplitude modulation (QAM)schemes, or the like, the modulation module modulates at least a part ofthe information to be transmitted as amplitude modulations of thetransmitted signal. As mentioned above, in the example of 16-QAMmodulation, when transmitting a bit stream by the transmitter, bitsequences of a predetermined length, e.g. four bits in the case of16-QAM or, in general, log₂(M) bits in M-QAM, are encoded as acorresponding one of a number of signal symbols which may be representedas a constellation of signal points in a the I/Q plane, as illustratedby the example of FIG. 2.

FIG. 2 shows a signal constellation with 16 signal symbols. The signalconstellation comprises M=16 signal points S₁ through S₁₆ in atwo-dimensional signal space, e.g. the I/Q components in a 16-QAM signalconstellation. Preferably, the signal points are distributed regularly,such that the distance to the nearest neighbours of each signal point isthe same. The signal points may take values that suit the implementationin question. The example of FIG. 2 corresponds to the signalconstellation proposed for the above-mentioned HSDPA. However,alternatively, other signal constellations may be chosen. In FIG. 2, 16different bit sequences 0000 through 1111, each consisting of log₂(M)=4bits, are mapped onto the signal points S₁-S₁₆. Preferably, the mappingof the bit sequences to the signal points is chosen such that the bitsequence of each signal point only differs from those of the nearestneighbours by one bit, thereby optimising the decoding performance. Forexample, in FIG. 2, signal point S₈ has three nearest neighbours, S₄,S₇, and S₁₂. The bit sequence of S₄, i.e. 0011, differs from thesequence 0010 of S₈ only at bit position 4, etc. Alternatively, othermappings may be chosen.

It is noted that, in the constellation of FIG. 2, the four bits of a bitsequence may be interpreted as sign (I), sign (Q), amplitude (I), andamplitude (Q), respectively. For example, when the first bit of a bitsequence is 1, the I-component of the corresponding signal symbol isnegative, i.e. the signal symbol is located in the region designated byreference numeral 201.

Otherwise, when the first bit of a bit sequence is 0, the I-component ofthe corresponding signal symbol is positive, i.e. the signal symbol islocated in the region designated by reference numeral 202. Similarly,when the third bit of a bit sequence is 0, the amplitude of theI-component is low, i.e. the signal symbol is located in the regiondesignated by reference numeral 205. Otherwise, when the third bit of abit sequence is 1, the amplitude of the I-component is high, i.e. thesignal symbol is located in the region designated by reference numeral206. Similarly, the second and fourth bits can be interpreted asindicating the sign and amplitude of the O-component, respectively: Whenthe second bit is 0, the Q-component is positive, i.e. the signal symbolis located in the region 203. When the second bit is 1, the Q-componentis negative, i.e. the signal symbol is located in the region 204. Whenthe fourth bit is 0, the Q-component is small, i.e. the signal symbol islocated in the region 207. When the fourth bit is 1, the Q-component islarge, i.e. the signal symbol is located in the region 208.

Hence, in the example of FIG. 2, only the third and fourth bits arerelated to the amplitude of the corresponding signal and, thus only thedecoding of the third and fourth bits is sensitive to amplitudedistortions, while the decoding of the initial two bits is notinfluenced by amplitude distortions. Thus, if the amplitude of thereceived signal is distorted, the soft values calculated for the thirdand fourth bit are unreliable and may, thus, cause a degraded receiverperformance. It is noted that this is a property of the specific codingof the example of FIG. 2, i.e. the specific mapping of bit sequences onsignal symbols. In other constellations different bits may depend onamplitude information. In yet other examples, a different number, insome examples even all, of the bits may depend on amplitude information.

FIG. 3 schematically shows a receiver according to an embodiment of theinvention. The receiver 102 receives a radio signal via a transmissionchannel 103. In one embodiment, the signal is a Code Division MultipleAccess (CDMA) signal using a spread spectrum technique.

The receiver 102 comprises a radio receiver circuit 301 for transformingthe received spread spectrum signal into a signal point r in a signalspace corresponding to a modulation comprising a constellation of Msignal symbols. The receiver 102 further comprises a channel decoder 304for decoding the received signal point r, e.g. a turbo decoder, a BCJRdecoder or a Viterbi decoder, etc., resulting in a decoded bit sequence305 comprising log₂(M) bits. The decoder 304 requires soft values as aninput. Hence, the receiver 102 further comprises a circuit 303 adaptedto calculate soft values for the log₂(M) bits of the received signalpoint r and to provide the calculated soft values to the decoder 304.Decoders utilising soft values are known as such in the art of digitalcommunications systems and will, thus, not be described in greaterdetail here.

For a received signal point r, the soft value calculation circuit 303calculates soft values L_(l,m) for the bits m=1, . . . , log₂(M) of thel-th received signal symbol, e.g. the bits of a 16-QAM symbol. The softvalues are fed to the decoder 304 together with the received radiosymbols. A soft value is calculated for each bit of every receivedsymbol. A soft value L_(l,m), may be defined as

$\begin{matrix}\begin{matrix}{L_{l,m} = {\log\frac{P\mspace{11mu}\left( {s_{l,m} = \left. 1 \middle| r \right.} \right)}{P\mspace{11mu}\left( {s_{l,m} = \left. 0 \middle| r \right.} \right)}}} \\{= {\log\frac{P\mspace{11mu}\left( {s_{l,m} = 1} \right)\mspace{11mu} P\mspace{11mu}\left( {\left. r \middle| s_{l,m} \right. = 1} \right)}{P\mspace{11mu}\left( {s_{l,m} = 0} \right)\mspace{11mu} P\mspace{11mu}\left( {\left. r \middle| s_{l,m} \right. = 0} \right)}}} \\{= {\log\frac{P\mspace{11mu}\left( {\left. r \middle| s_{l,m} \right. = 1} \right)}{P\mspace{11mu}\left( {\left. r \middle| s_{l,m} \right. = 0} \right)}}}\end{matrix} & (1)\end{matrix}$where S_(l,m) is the m-th bit in the l-th signal symbol represented bythe transmitted signal, and P(S_(l,m)=il r), i=0,1, are the a posterioriprobabilities of the bit S_(l,m) where r is the received signal. Hence,large negative values of L_(l,m) correspond to a high likelihood of bitS_(l,m) being zero, while large positive values of L_(l,m) correspond toa high likelihood of the bit S_(l,m) being one. A value L_(l,m) close tozero corresponds to an unreliable bit and, in particular, L_(l,m)=0corresponds to an unreliable bit where S_(l,m)=0 and S_(l,m)=1 areequally probable.

It is noted that the second equality in eqn. (1) assumes that S_(l,m)=1and S_(l,m)=0 are equally probable in the chosen alphabet. Otherwise,the overall ratio of probabilities should be taken into consideration inthe following. However, this would only give rise to a constant factor.Hence, L_(l,m) corresponds to a log-likelihood ratio of probabilities.The probabilities P (r|S_(l,m)=i) in eqn. (1) may be written as

$\begin{matrix}{{{P\left( {S_{l.m} = 1} \right)} = {{c{\sum\limits_{S,{ɛ1}_{1,m}}^{\;}\;{P\left( {r,s_{l,m}} \right)}}} = 0}},1.} & (2)\end{matrix}$

Here, s_(l) is the l-th received symbol, A_(l,m)={S_(l)|S_(l,m)=i} isthe set of signal symbols in the symbol constellation having the value iat bit position m, and c is a constant factor.

Hence, the calculation of the above probability involves a summationover M/2 terms each including a joint probability P (r,S_(l,m)). This isa computationally expensive task, especially if M is large, e.g. M=16 oreven M=64.

In many applications, the above soft values L_(l,m) may be approximatedby

$\begin{matrix}{L_{l,m} = {{\log\frac{{\max\;{P\left( {r,s_{i,m}} \right)}}{s_{i}ɛ\; A_{l,m}}}{{\max\;{P\left( {r,s_{i,m}} \right)}}{s_{i}ɛ\; A_{o,m}}}} = {\log\frac{P\left( r \middle| {\overset{\_}{s}}_{1,l,m} \right)}{P\left( r \middle| {\overset{\_}{s}}_{0,l,m} \right)}}}} & (3)\end{matrix}$where Ŝ_(l,m), i=0, 1, are the signal points that result in the largestcontribution to the sums in eqn. (2). Hence, in the calculation of theprobabilities, the sums over M/2 terms are approximated by the theirrespective dominant terms, according to

$\begin{matrix}{{\log{\sum\limits_{s\; ɛ\; A_{l,m}}^{\;}\;{P\left( {r,s} \right)}}} \approx {\log_{s\; ɛ\; A_{l,m}}^{\max}{{P\left( {r,s} \right)}.}}} & (4)\end{matrix}$

The above approximation is often referred to as the “max log MAP”approximation which yields a good approximation in cases where the abovesums are dominated by one term, as for example in the case of Gaussiannoise when the signal to noise ratio (SNR) is large. The aboveprobabilities depend on the distances between the received signal r andthe respective signal points. For example, in the case of additivezero-mean Gaussian noise with variance σ₂, the log-likelihood ratio ofeqn. (3) may be expanded as

$\begin{matrix}\begin{matrix}{L_{l,m} = {\log\frac{\sigma^{- 2}\exp\mspace{11mu}\left( {{- {{r - {\hat{s}}_{1,l,m}}}^{2}}/\sigma^{2}} \right)}{\sigma^{- 2}\exp\mspace{11mu}\left( {{- {{r - {\hat{s}}_{0,l,m}}}^{2}}/\sigma^{2}} \right)}}} \\{= {\sigma^{- 2}\mspace{11mu}{\left( {{{r - {\hat{s}}_{0,l,m}}}^{2} - {{r - {\hat{s}}_{1,l,m}}}^{2}} \right).}}}\end{matrix} & (5)\end{matrix}$

Hence, the soft value L_(l,m) is approximated by the scaled differenceof the squared distances to the closest signal points having oppositebit values at position m.

In a preferred embodiment, for a given bit m of a received signal symbolS_(l), the likelihood ratio in equation (5) is obtained by firstidentifying the closest signal point Si, by determining the distance δ₁to that signal point and, subsequently determining the distance δ₂ tothe closest signal point having a bit value at position m which isdifferent from the corresponding bit value of S_(l). Hence, thisembodiment provides a computationally inexpensive calculation of softvalues.

Preferably, the closest signal point having opposite bit value thanS_(l) may be looked up in a look-up table. In another preferredembodiment, the distance to the closest signal point having opposite bitvalue than S_(l) may be approximated by the known distance between S_(l)and the closest signal point having opposite bit value than S_(l). Thisdistance, in turn, may be looked up in a look-up table of pre-calculateddistances, thereby further reducing the computational complexity of thesoft value calculation.

It is noted that, preferably, in the above estimation of the reliabilityvalues, a proper scaling of the signal points in the signalconstellation should be taken into consideration, as will be describedin greater detail below. However, for the purpose of the presentdiscussion, it may be assumed that the signals in eqn. (5) are properlyscaled.

According to the invention, the receiver 102 further comprises a controlunit 302 which provides a reliability indication to the soft valuecalculation circuit 303. The reliability indication provides anindication as to whether the amplitude information of the receivedsignal symbol r is reliable. As will be described in greater detailbelow, this information is used by the soft value calculation circuit inthe calculation of the soft values, thereby providing an improved softvalue calculation.

In general, the soft value calculation should calculate

$\begin{matrix}{L_{l,m} = {\log\frac{P\mspace{11mu}\left( {{s_{l,m} = \left. 1 \middle| r \right.},A} \right)}{P\mspace{11mu}\left( {{s_{l,m} = \left. 0 \middle| r \right.},A} \right)}}} & (6)\end{matrix}$where A represents a priori amplitude information, e.g. that thereceiver has entered compression. Hence P (S_(l,m)=i|r, A) representsthe probability of a bit value S_(l,m)=i, given that the signal r hasbeen received and given the amplitude information A.

Even though the receiver may not be able to restore the distortedsignal, it may improve the error rate of the decoded signal, if thereceiver is aware of the distortion. For example, the error rate of thedecoded signal may be measured as a Block Error Rate (BLER).

As the true distribution of A may be difficult to obtain, in someembodiments it may be useful to rely on approximations to the aboveequation (6). An embodiment of such an approximation will be describedin greater detail below.

FIG. 4 shows a more detailed block diagram of a receiver according to afirst embodiment of the invention. The receiver 102 comprises an antenna401 and a front-end receiver 402 for receiving a radio signal. Thefront-end receiver receives the radio signal and down-converts it to abaseband signal. The receiver 102 further comprises an amplifier 403which receives the down-converted signal from the front-end receiver 402and scales it in order to better utilise the dynamic range of thesubsequent digital domain of the receiver. In the embodiment of FIG. 4,this is achieved by an automatic gain control circuit 410. The receiver102 further comprises an analog-to-digital converter 404 for convertingthe scaled radio signal into a digital representation. The automaticgain control unit 410 receives the digital signal and controls the gainof the amplifier 403 in order to obtain a proper scaling. The scalingdepends on the absolute received signal strength which, in turn, dependson the distance between the receiver and the transmitter and on thecurrent fading situation. While the distance between the receiver andthe transmitter typically varies slowly, the fading may change quiterapidly, typically on a time scale less than one slot, i.e. 0.67 ms, inWDCMA. Since an automatic gain control as such is known in the art ofdigital communications systems, it will not be described in greaterdetail here.

The receiver 102 further comprises a channel estimator and RAKE receiver405 which receives the digital signal as an input. The RAKE receiveruses several baseband correlators to individually process several signalmultipath components. The correlator outputs are combined to achieveimproved communications reliability and performance (see e.g. “DigitalCommunications” 4th Edition, by John G. Proakis, McGraw-Hill, 2000). TheRAKE receiver generates the signal symbol r to be decoded. The circuit405 further comprises a channel estimator, e.g. implementing anysuitable channel estimation technique known in the art. The channelestimator receives the digital baseband representation of the receivedradio signal and provides an estimate of the radio channel based on areference or pilot channel. In one embodiment, the channel estimatoridentifies up to N different radio paths or channel taps h_(r)=(h_(r1),. . . h_(rN)). The channel estimator further provides a set of complexcombiner weights w=[w₁ W₂ . . . W_(N)] which include a channel estimateand, in some embodiments, an interference estimate. For example, theweights may be determined according to an optimisation criterion, suchas maximising the received signal to interference ratio (SIR).Typically, the channel estimate is computed on a slot basis, i.e. achannel estimate represents an average of the actual channel over onetime slot.

In the following the scaling of the signal symbols is described in moredetail, in order to improve readability and to describe a possibleimplementation. As mentioned above, in the calculation of the softvalues, the signal symbols should be scaled properly. In the followingit is assumed that the channel estimator estimates the channel gain onthe basis of a reference or pilot channel hr which has a channel gainthat may be different from the actual gain of the traffic channel, e.g.a HS-DSCH. The gain difference between the reference channel and thetraffic channel may be denoted with g. Hence, the received signal rafter the RAKE receiver 405 may be expressed asr=gw ^(H) h _(r) s+n,where w^(H) is the Hermitian conjugate of w, w^(H)hr denotes an innerproduct, s is the transmitted symbol and n is a noise term, e.g.representing additive white Gaussian noise (AWGN). The gain parameter gis assumed to be signaled to the receiver, w is selected by the combinerin the receiver, and h_(r) are the channel taps. Hence, at the receiver,the reference signal symbols S₁ . . . S_(M) may be scaled appropriately,according toS_(j)=gw^(H)h_(r)S_(j), j=1, . . . , M.  (7)

When this scaling is taken into consideration, the above log-likelihoodratio may be written asL _(l,m) =K·(|{tilde over (r)}−s _(0,l,m)|² −|{tilde over (r)}−{tildeover (s)} _(1,l,m)|²)  (8)i.e. with the properly scaled signals

$\begin{matrix}{{\overset{\sim}{r} = \frac{r}{{gw}^{H}h_{r}}}{{\overset{\sim}{s}}_{i,l,m} = {{\frac{{\hat{s}}_{i,l,m}}{{gw}^{H}h_{r}}\mspace{14mu} m} = 1}},\ldots\mspace{11mu},{\log_{2}\;(M)},{i = 0},1,} & (9)\end{matrix}$and whereK=(gw ^(H) h _(r))²/σ²  (10)is a constant which depends on the signal to noise ratio.

The output of the RAKE receiver of circuit 405 is fed to the soft valuecalculation circuit 406 which calculates the soft values as wasdescribed in connection with eqns. (5) and (8). The calculated softvalues are fed to the decoder 408 for decoding the received signalsymbol into the corresponding bit sequence as described above. If thedecoding is performed successfully, the decoder forwards the acceptedbit sequence 416 for further processing in the receiver. Otherwise, thedecoder generates a signal 415 indicating that the signal symbol couldnot be reliably decoded. This signal may be fed back to a transmittercircuit (not shown) causing the transmitter to return a NACK message tothe transmitter that has sent the unreliable signal symbol, therebyrequesting a re-transmission.

According to the invention, the receiver 102 further comprises a controlunit 302 which receives information about gain changes from theautomatic gain control unit 410. Based on this information, the controlunit 302 determines whether any amplitude changes have occurred within atime slot. If the control unit detects such a change, it forwards thisinformation to the soft value calculation circuit 406 which uses thisinformation to adjust the calculated soft values accordingly.

Typically, in a digital communications system, a number of time slotsare comprised in a transmission time interval (TTI), and a receiverde-interleaves and decodes the time slots of one TTI together. Forexample, in HSDPA the TTI length is 3 slots, i.e. three time slots arede-interleaved and decoded together.

Hence, according to the invention, the control unit 302 may detect thefraction of time slots within a TTI with unreliable amplitudeinformation. In the embodiment of FIG. 4, the control unit detects ifthe fraction of soft values affected by amplitude distortions is largerthan a predetermined threshold, e.g. larger than 50%. If this is thecase, the corresponding signal is not forwarded to the decoder and adecoding is not even attempted, as illustrated by the switch circuit 407controlled by the control unit 302 via control signal 412. Instead, aNACK report 414 is generated by the control unit and returned to thetransmitter. It is an advantage of this embodiment that amplitudedistorted TTIs may be detected at an early stage within the receiver,thereby improving the overall throughput of the receiver by avoidingfutile decoding attempts.

FIG. 5 shows a more detailed block diagram of a receiver according to asecond embodiment of the invention. The receiver 102 of this embodimentcomprises an antenna 501, a front-end receiver 502, an amplifier 503,and A/D converter 504, a channel estimator and RAKE receiver 505, a softvalue calculation circuit 506 and a decoder 508. The above componentscorrespond to the respective components described in connection with theembodiment of FIG. 4.

According to this embodiment, the receiver comprises a control unit 302which receives the output of the RAKE receiver and determines changes inthe amplitude, e.g. based on the channel estimate of the channelestimator.

For example, in one embodiment quantization effects are treated in thecombiner of the RAKE receiver 505 by using a time-varying scaling. Thechange of the scaling leads to temporary amplitude distortions. Hence,since information about this change of scaling is available from thereceiver, this information can be used by the control circuit 302 toadjust the calculated soft values for affected bits.

As described above, the control unit 302 feeds the determinedinformation about amplitude distortions to the soft value calculationcircuit 506 where it is used to adjust the calculated soft values.

FIG. 6 shows a flow diagram of a method according to an embodiment ofthe invention. In the following description reference will also be madeto the block diagram of FIG. 5. In step 601, a slot comprisingtransmitted data is received. In step 602, the soft values arecalculated, e.g. as described in connection with eqn. (5). In step 603,the control unit 302 detects whether the received amplitude informationis reliable. If yes, the process continues at step 605. If not, theprocess continues at step 604 where the soft values that are affected bythe detected amplitude distortion are adjusted before proceeding at step605. For example the soft values may be multiplied by a scaling factor0≦η<1, e.g. η=0.1, thereby reducing the absolute value of the softvalues. At step 605, the signal is decoded on the basis of thecalculated soft values, resulting in an acceptance of the receivedsymbol as reliable or, if the decoding step failed, a rejection of thesymbol as unreliable.

FIG. 7 shows a flow diagram of a method according to another embodimentof the invention. The flow diagram of FIG. 7 illustrates the decoding ofsignal symbols received within a transmission time interval comprising anumber of time slots. In the following description reference will alsobe made to the block diagram of FIG. 4. In step 701, the next slot of atransmission time interval is received. In step 702, it is detectedwhether, during the current time slot, a change in gain settings of theamplifier 403 has taken place caused by the gain control unit 410. Thismay be detected by comparing the times at which gain changes are causedby the gain control circuit with the slot boundaries. If a change ingain has taken place, the process proceeds at step 704 where the softvalues which are affected by amplitude distortions are set to zero. Forexample, in the symbol constellation of FIG. 2, the effected soft valuesare the soft values L_(l,3) and L_(l,4) of the third and fourth bit of asymbol, respectively. Hence, even though the actual distributions ineqn. (6) may not be known or at least difficult to compute, the aboveapproach provides a simple approximation which has proven useful in areceiver as shown in FIG. 4.

If no change has taken place, it is detected, in step 703, whether theamplifier has reached saturation. If this is the case, the amplitudeinformation is considered unreliable as well and the process proceeds atstep 704. Otherwise, no adjustment of the soft values is performed andthe process continues at step 705.

At step 705, it is determined whether the total number of soft valuesaffected within the current transmission time interval has reached apredetermined threshold, e.g. 50% of all soft values. If this is thecase, the receiver generates a NACK report and terminates processing ofthe time slots of the current transmission time interval withoutattempting to decode any of the corresponding signal symbols.

Otherwise, the process proceeds at step 706, where it is determinedwhether all time slots of the current transmission time interval havebeen received. If not, the process returns to step 701 and processes thenext time slot.

If all time slots of a transmission time interval have been received andthe transmission time interval has not been rejected in step 705, thesoft values of the unaffected bits, i.e. the soft values which have notbeen set to zero, are calculated in step 707, as was described inconnection with eqns. (5) and (8).

Subsequently, the process proceeds at step 708 where the signal symbolsof the current transmission time interval are decoded as describedabove, resulting in an acceptance or a rejection of the decoded signal.

FIG. 8 illustrates the method of FIG. 7 with reference to an example ofa communications signal. FIG. 8 illustrates a received signalcorresponding to a time frame comprising 9 time slots 811, 812, 813,814, 815, 816, 817, 818, and 819. In FIG. 8, it is assumed that atransmission time interval comprises 3 time slots, as in HSDPA, and thatthe received signal is modulated according to 16-QAM with a signalconstellation as illustrated in FIG. 2. Hence, the depicted framecomprises TTIs 807, 808, and 809. The received signal strength isindicated by the solid line 801. Hence, in the example of FIG. 8, thesignal strength is assumed to decrease towards the middle of the frameand to increase again towards to end of the frame. Hence, in thisexample, the amplitude varies considerably on a time scale correspondingto a TTI, in particular during TTI 808. As mentioned above, such achange in amplitude may be caused by a change in the fading situation.

As described above, in the receiver the above amplitude changes causethe gain settings in the amplifier to change, e.g. by a gain controlcircuit as described in connection with FIG. 4. Line 802 illustrates thegain setting corresponding to the received signal 801 and, inparticular, changes in the gain settings indicated by reference numerals821, 822, 823, 824, 825, 826, 827, and 828. Hence, during time slots813, 814, 816, and 817, the gain is changed. In FIG. 8 it is furtherassumed that the channel is estimated once per time slot using standardaveraging, resulting in channel estimates h₁, h₂, h₃, h₄, h₅, h₆, h₇,h₈, and h₉.

The dotted line 803 illustrates the maximum gain level of the amplifier.In the example, of FIG. 8, it is assumed that during TTI 808, the gainreaches the maximum level as indicated by reference numeral 804. Thissituation is likewise detected by the control unit 302 of FIG. 4 andresults in the time slot 815 to be marked unreliable as well.

Hence, the time slots 813, 814, 815, 816, and 817 are affected byamplitude distortions, as indicated by reference numeral 830.

In TTI 807, only the last time slot, i.e. time slot 813 is affected bythe amplitude distortions. According to the method of FIG. 7, thecorresponding affected soft values are set to zero, i.e. assuming theconstellation of FIG. 2 the soft values L_(l,3) and L_(l,4), asindicated by reference numeral 805. Hence, in this example, ⅓ or thetime slots of the TTI 807 are affected and ⅙ of the soft values.Likewise, in TTI 809, only the first time slot, i.e. time slot 817 isaffected by the amplitude distortions and, accordingly, the soft valuesL_(l,3) and L_(l,4), of time slot 817 are set to zero as indicated byreference numeral 806. Again ⅓ or the time slots of the TTI 809 areaffected and ⅙ of the soft values.

In the TTI 808, on the other hand. i.e. the TTI where the fading dip ofthe signal strength 801 is most pronounced, all time slots are affectedby amplitude distortions causing 50% of the soft values of TTI 808 to bedistorted. According to the method of FIG. 7, the control unit 302 ofFIG. 4 may detect this situation prior to the decoding of thecorresponding signal, and has the possibility of triggering a NACKreport without starting the decoding process.

It is noted that the invention was described in connection with softvalues defined as a log-likelihood ratio indicating a reliability valuefor the bit values of a received sequence.

However, other definitions of likelihood values may be used within thescope of the invention.

It is further noted that the signal constellation of FIG. 2 is merelyused as an example. The calculation of soft values according to theinvention is not limited to this signal constellation.

In general, it is noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims, e.g. the skilled person may combinefeatures illustrated in connection with individual embodiments.

1. A method of decoding a communications signal in a digitalcommunications system, wherein the communications signal is modulatedaccording to a modulation scheme including amplitude information, themethod comprising: receiving a communications signal by a receivermodules; generating a likelihood value for a received communicationssignal, wherein the step of generating the likelihood value furthercomprises generating the likelihood value on the basis of the providedreliability indication of the amplitude information, wherein thereliability indication is provided by the receiver module; decoding thecommunications signal based on at least the generated likelihood value;and providing a reliability indication of the amplitude informationconveyed by the received communications signal.
 2. The method accordingto claim 1, wherein the step of receiving the communications signalfurther comprises scaling the communications signal by an amplifier andthe step of providing the reliability indication by the receiver modulecomprises generating the reliability indication on the basis of a gainsetting of said amplifier.
 3. The method according to claim 1, whereinthe communications signal is modulated according to a quadratureamplitude modulation scheme.
 4. A method of decoding a communicationssignal in a digital communications system, wherein the communicationssignal is modulated according to a modulation scheme including amplitudeinformation, the method comprising generating a likelihood value for areceived communications signal, wherein the step of generating thelikelihood value further comprises generating the likelihood value onthe basis of the provided reliability indication of the amplitudeinformation, the step of generating the likelihood value on the basis ofthe provided reliability indication further comprising determiningwhether an amplitude change by a predetermined magnitude has occurredwithin a predetermined time period; decoding the communications signalbased on at least the generated likelihood value; and providing areliability indication of the amplitude information conveyed by thereceived communications signal.
 5. The method according to claim 4,wherein the predetermined time period corresponds to a time slot forcommunicating a bit sequence.
 6. A method of decoding a communicationssignal in a digital communications system, wherein the communicationssignal is modulated according to a modulation scheme including amplitudeinformation, the method comprising generating a likelihood value for areceived communications signal, wherein the step of generating thelikelihood value further comprises generating the likelihood value onthe basis of the provided reliability indication of the amplitudeinformation wherein the step of generating the likelihood value on thebasis of the provided reliability indication of the amplitudeinformation further comprises adjusting the likelihood value to a valuecorresponding to higher uncertainty of a predetermined bit value if anamplitude change has occurred and if said bit value is encoded usingamplitude information; decoding the communications signal based on atleast the generated likelihood value; and providing a reliabilityindication of the amplitude information conveyed by the receivedcommunications signal.
 7. A method according to claim 6, wherein themodulation scheme is 16-QAM wherein each signal symbol comprises fourbits and where two predetermined bits of said four bits depend onamplitude information; and the method further comprises adjusting thelikelihood values of said two predetermined bits to a valuecorresponding to higher uncertainty, if a change in amplitude isdetected during the time slot for communicating said four bit sequence.8. A method of decoding a communications signal in a digitalcommunications system, wherein the communications signal is modulatedaccording to a modulation scheme including amplitude information, themethod comprising generating a likelihood value for a receivedcommunications signal, wherein the step of generating the likelihoodvalue further comprises generating the likelihood value on the basis ofthe provided reliability indication of the amplitude information;decoding the communications signal based on at least the generatedlikelihood value; providing a reliability indication of the amplitudeinformation conveyed by the received communications signal; andrejecting all received signal symbols received within a time intervalcomprising a predetermined number of consecutive slots, if a change inamplitude by a predetermined magnitude is detected in more than apredetermined fraction of slots of said number of consecutive slots. 9.A method of decoding a communications signal in a digital communicationssystem, wherein the communications signal is modulated according to amodulation scheme including amplitude information, the method comprisinggenerating a likelihood value for a received communications signal,wherein the step of generating the likelihood value further comprisesgenerating the likelihood value on the basis of the provided reliabilityindication of the amplitude information; decoding the communicationssignal based on at least the generated likelihood value; and providing areliability indication of the amplitude information conveyed by thereceived communications signal, wherein the communications signal is acommunications signal of a High Speed Downlink Packet Access of a 3GPPWideband Code Division Multiple Access.
 10. An arrangement for decodinga communications signal in a digital communications system, where thecommunications signal is modulated according to a modulation schemeincluding amplitude information, the arrangement comprising: a receivermodule for receiving the communications signal; processing means adaptedto generate a likelihood value for a received communications signal,wherein the processing means is further adapted to generate thelikelihood value on the basis of the provided reliability indication ofthe amplitude information, the receiver module being adapted to providethe reliability indication; a decoder for decoding the communicationssignal based on at least the generated likelihood value; and means forproviding a reliability indication of the amplitude information conveyedby the received, communications signal.
 11. The arrangement according toclaim 10, wherein the arrangement further comprises an amplifier forscaling the received communications signal; and wherein the receivermodule is further adapted to generate the reliability indication on thebasis of a gain setting of said amplifier.
 12. The arrangement accordingto claim 10, wherein the communications signal is modulated according toa quadrature amplitude modulation scheme.
 13. The arrangement accordingto claim 10, as implemented in a cellular communications system.
 14. Thearrangement according to claim 10, wherein the arrangement is comprisedin a mobile terminal of the cellular communications system.
 15. Thearrangement according to claim 10, wherein the arrangement is comprisedin a base station of the cellular communications system.
 16. Thearrangement according to claim 10, as implemented in a device forreceiving a communications signal.
 17. The arrangement according toclaim 16, wherein the device further comprises a decoder adapted toreceive an input signal from the arrangement indicative of thedetermined reliability value.
 18. The arrangement according to claim 16wherein the device is a mobile terminal.
 19. An arrangement for decodinga communications signal in a digital communications system, where thecommunications signal is modulated according to a modulation schemeincluding amplitude information, the arrangement comprising: processingmeans adapted to generate a likelihood value for a receivedcommunications signal, wherein the processing means is further adaptedto generate the likelihood value on the basis of the providedreliability indication of the amplitude information wherein theprocessing means is further adapted to determine whether an amplitudechange by a predetermined magnitude has occurred within a predeterminedtime period; a decoder for decoding the communications signal based onat least the generated likelihood value; and means for providing areliability indication of the amplitude information conveyed by thereceived, communications signal.
 20. The arrangement according to claim19, wherein the predetermined time period corresponds to a time slot forcommunicating a bit sequence.
 21. An arrangement for decoding acommunications signal in a digital communications system, where thecommunications signal is modulated according to a modulation schemeincluding amplitude information, the arrangement comprising: processingmeans adapted to generate a likelihood value for a receivedcommunications signal, wherein the processing means is further adaptedto generate the likelihood value on the basis of the providedreliability indication of the amplitude information wherein theprocessing means is further adapted to adjust the likelihood value to avalue corresponding to higher uncertainty of a predetermined bit valueif an amplitude change has occurred and if said bit value is encodedusing amplitude information; a decoder for decoding the communicationssignal based on at least the generated likelihood value; and means forproviding a reliability indication of the amplitude information conveyedby the received, communications signal.
 22. The arrangement according toclaim 21, wherein the modulation scheme is 16-QAM wherein each signalsymbol comprises four bits and where two predetermined bits of said fourbits depend on amplitude information; and the processing means isfurther adapted to adjust the likelihood values of said twopredetermined bits to a value corresponding to higher uncertainty, if achange in amplitude is detected during the time slot for communicatingsaid four bit sequence.
 23. An arrangement for decoding a communicationssignal in a digital communications system, where the communicationssignal is modulated according to a modulation scheme including amplitudeinformation, the arrangement comprising: processing means adapted togenerate a likelihood value for a received communications signal,wherein the processing means is further adapted to generate thelikelihood value on the basis of the provided reliability indication ofthe amplitude information, wherein the processing means is furtheradapted to reject all received signal symbols received within a timeinterval comprising a predetermined number of consecutive slots, if achange in amplitude by a predetermined magnitude is detected in morethan a predetermined fraction of slots of said number of consecutiveslots; a decoder for decoding the communications signal based on atleast the generated likelihood value; means for providing a reliabilityindication of the amplitude information conveyed by the received,communications signal.
 24. An arrangement for decoding a communicationssignal in a digital communications system, where the communicationssignal is modulated according to a modulation scheme including amplitudeinformation, the arrangement comprising: processing means adapted togenerate a likelihood value for a received communications signal,wherein the processing means is further adapted to generate thelikelihood value on the basis of the provided reliability indication ofthe amplitude information; a decoder for decoding the communicationssignal based on at least the generated likelihood value; and means forproviding a reliability indication of the amplitude information conveyedby the received, communications signal, wherein the communicationssignal is a communications signal of a High Speed Downlink Packet Accessof a 3GPP Wideband Code Division Multiple Access.
 25. The arrangementaccording to claim 10, further comprising: a receiver for receiving acommunication signal; an amplifier for scaling the receivedcommunications signal according to a predetermined amplifier gain; again control module for controlling the amplifier gain according to areceived signal strength, the gain control module being adapted to feeda gain control signal to the amplifier; and a control unit forgenerating amplitude information, the control unit being adapted toreceive the gain control signal from the gain control unit and togenerate an amplitude information signal; and that the processing meansis adapted to receive the amplitude information signal from the controlunit.